1. Field of the Invention
This invention relates to an improved process to make an alpha alumina monohydrate powder which is useful as a shear-rate thinning viscosifier in drilling fluids.
2. Description of the Previously Published Art
U.S. Pat. No. 4,240,915 to Block describes water-based, clay-free drilling fluids for use in drilling bore holes into subterranean formations which contain water, a water-loss inhibitor, a weighting agent and aluminum hydroxide as a viscosifying agent. The aluminum hydroxide viscosifying agent is prepared in an aqueous medium by contacting and mixing under a high degree of agitation an acid reactant and a base reactant wherein at least one of the acid or base reactants contains aluminum. The acidic reactant can be either an inorganic acid or a salt of a strong acid and a weak base. These aqueous compositions contain a significant amount of water and are expensive to transport.
To reduce the water weight, attempts have been made to dry the aluminum hydroxide. However, when it is dried, the aluminum hydroxide powder does not readily disperse back into water to yield the original shear-rate thinning material. Thus this dried form would not be suitable for its intended use at a well site where it should be able to disperse in water to form a shear-rate thinning drilling fluid.
U.S. Pat. No. 4,244,835 to Block describes one way to solve this problem of redispersing the dried aluminum hydroxide powder in water. There it was necessary to add a base such as a water soluble alkali metal hydroxide, carbonate or oxide to the mixture and it was further necessary to employ high shear mixing. This technique, however, not only requires the presence of additional chemicals which increase the cost of the drilling fluid but it also requires high shear mixing. In addition the handling of the caustic is hazardous and it may result in the pH of the reaction mixture being too high which would then require readjusting with an acidic material.
In U.S. patent application Ser. No. 391,112 filed June 23, 1982 and now abandoned to Block a relatively dry solid AlO(OH) is formed by heat aging the reactants at a temperature of from about 60.degree. C. to 100.degree. C. for a period of from about 2 to 7 hours. There the heat aging is shown to be critical since AlO(OH) produced without the heat aging and dried to an AlO(OH) content of at least 35% by weight could not be redispersed in water. The filter cake was washed to remove by-product salts, but no special significance was attributed to the washing step.
3. Objects of the Invention
It is an object of this invention to produce an alpha alumina monohydrate gel which can be dispersed in water to yield a highly shear-rate thinning fluid with an n value in the power law model (to be discussed below) of less than 0.3 and with an increased K value without the aid of large quantities of acids, bases, excessive high shear mixing or heat aging.
It is a further object of this invention to form a shear-rate thinning alpha alumina monohydrate in an aqueous system by reacting under a high degree of agitation an acid reactant and a base reactant wherein at least one of the acid or base reactants contains aluminum and to then utilize a salt removal treatment to produce an alpha alumina monohydrate form of aluminum hydroxide which can be dispersed in water to form a shear-rate thinning material without the need for any large quantities of auxiliary dispersing agents, any expensive high shear mixing equipment, or time consuming thermal aging.
It is a further object of this invention to produce an aluminum hydroxide viscosifying material which can be dried and then dispersed to yield a shear-rate thinning viscosifier with a higher K value per gram of AlO(OH) than previously obtained.
These and further objects will become apparent as the description of the invention proceeds.